An optical wavelength division multiplexing (WDM) system for transmitting light of multiple wavelengths through a single transmission line has been used to realize high-capacity transmission and/or simultaneous bidirectional transmission. In a WDM system, there are a variety of multi/demultiplexers for combining and/or separating multiplexed light, and a low-cost device is required for such multi/demultiplexers used in subscriber (access) systems.
FIG. 14 shows a conventional wavelength multi/demultiplexer 500.
The conventional wavelength multi/demultiplexer 500 is a low-cost device, which combines and/or separates two wavelengths of 1.3 μm and 1.55 μm (see Patent Document 1, for example). The “wavelength multi/demultiplexer” for use in optical communications is a device that combines signals with different wavelengths together and/or separates them apart.
The conventional wavelength multi/demultiplexer 500 has single-mode optical waveguides 2, 3 and 2′, a groove 4 provided at a position where the optical waveguides 2 and 3 intersect each other, and a dielectric multilayer filter 5 inserted into the groove 4. The filter 5 has a reflection band at the 1.55 μm band and a pass band at the 1.31 μm band.
The dielectric multilayer filter 5 is arranged to be perpendicular to the bisector of the intersection angle between the optical waveguides 2 and 3, and in a manner that its reflective surface is positioned at the intersection point of the optical waveguides 2 and 3.
As such, a geometric reflective structure is provided by the optical waveguides 2, 3 and the dielectric multilayer filter 5, and the optical waveguide 2′ is arranged for the light passed through the dielectric multilayer filter 5. In this way, for the multiplexed light of 1.31 μm and 1.55 μm wavelengths that travel through the optical waveguide 2 via an optical fiber (not shown), the 1.55 μm light is reflected at the dielectric multilayer filter 5 and output to the optical waveguide 3. At the same time, the 1.31 μm light is passed through the dielectric multilayer filter 5 and output to the optical waveguide 2′.
In this configuration, since the optical waveguide 3, into which the 1.55 μm light reflected at the dielectric multilayer filter 5 is coupled, is a single-mode optical waveguide, how the coupling loss is to be reduced is an important issue. To solve this issue, a setting position of the dielectric multilayer filter 5, an intersection angle between the optical waveguides 2 and 3, and a maker position for high-precision groove processing have been optimized, and a multi/demultiplexer with a required loss has been realized (see Patent Document 1, for example).
For reference's sake, in the conventional wavelength multi/demultiplexer 500, the optical waveguide 2′ is branched into a Y-shape, and for each of the branched optical waveguides, a laser diode or a photodiode is provided fabricating a transmitter/receiver module.
It should be note that the Y-shape branched optical waveguides, laser diode, and photodiode are omitted in FIG. 14.
Recently, there have been advances in the diversification of services in the access systems, and the wavelength spacing to be separated is becoming narrower. For example, in the PON (Passive Optical Network) system for single-fiber bidirectional communications, the 1480-1580 nm band used for downstream signals are divided into two bands of 1480-1500 nm and 1550-1560 nm. It has then been proposed to assign the latter band to another future service, such as vide delivery (see Non-Patent Document 1, for example).
According to this prior art embodiment, a demultiplexer for separating the 1480-1500 nm band and the 1550-1560 nm band is required to have a performance that separates the narrowest spacing of two wavelengths of 1500 nm and 1550 nm.
Also, as another prior art embodiment, in an optical line testing system using a different wavelength from the communication wavelength, the test light wavelength of 1650 nm is used relative to the upper limit wavelength of 1625 nm in the communication wavelength band (see Patent Document 2, for example). In this case, it is required to separate the signal light and the test light which are adjacent with 25 nm.
If a wavelength multi/demultiplexer for the two wavelengths disposed at such a narrow wavelength spacing can be realized with a configuration using a conventional intersectional optical waveguide, it is advantageous for cost reduction.
When constructing a wavelength multi/demultiplexer based on the above configuration, due to the incident light to the dielectric multilayer filter 5 being divergent light, the wavelength response, that is slope of the transmission spectrum in the wavelength region from a pass band to a reflection band in the resulting multi/demultiplexing characteristics is degraded. Therefore for narrow separation wavelength spacing, the wavelength response degradation in the pass band can not be ignored. Also, it is required to increase the thickness of the dielectric multilayer to narrow the separation spacing, which leads to affect spectral degradation due to the divergent light furthermore.
FIG. 15 shows a characteristic of the wavelength multiplexer/demultiplexer 500 in the above prior art embodiment.
The inventors have studied with their prototypes by setting the refractive index difference of optical waveguides at a practical lower limit of about 0.3% and found a spectral degradation, as shown in FIG. 15, in the reflection path from the optical waveguide 2 to the optical waveguide 3, which hindered the realization of a wavelength multi/demultiplexer.
This spectral degradation has a peak P of a minimum loss around the edge wavelength of the reflection band and shows an increase in loss on its longer-wavelength side, which cannot be explained from the characteristics of the dielectric multilayer filter 5.
In addition, in the multi/demultiplexing spectrum, a problem exists in that the wavelength response around the edge wavelength from the pass band to the reflection band may not be good enough.
Accordingly, it is an object of the present invention to provide a wavelength multi/demultiplexer with intersectional optical waveguides having no spectral degradation and good wavelength response even for two narrowly spaced wavelengths.    Patent Document 1: Japanese Patent Application Laid-open No. 8-190026 (1996)    Patent Document 2: Japanese Patent Application Laid-open No. 2002-368695    Non-Patent Document 1: NTT Technical Journal, Vol. 15, No. 1, January 2003, pp. 24-27